Unfortunately koji deals with a number of overloaded terms. We try to be internally consistent, but can cannot control how others chose to talk about koji or build tools in general.
In Koji, a buildroot is the chroot environment on a build host that is used to perform various tasks (mainly builds). Since Koji generates fresh buildroots for each build there are a *lot* of buildroots.
Buildroots do not have any sort of name. They only have a numeric id. You seem to want to refer to a build root by a tag name, but a tag can correspond to many, many buildroots (or none at all).
While the list-buildroot command could certainly use a few improvements, the one you're asking for is nonsensical.

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